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Re: [Qemu-block] [PATCH v5 18/27] mirror: Disable image locking on targe

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v5 18/27] mirror: Disable image locking on target backing chain
Date: Fri, 3 Jun 2016 15:24:03 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

On 03.06.2016 08:32, Fam Zheng wrote:
> On Tue, 05/24 20:20, Max Reitz wrote:
>> On 17.05.2016 09:35, Fam Zheng wrote:
>>> In sync=none the backing image of s->target is s->common.bs, which could
>>> be exclusively locked, the image locking wouldn't work here.
>> Why is the backing image s->common.bs when the bdrv_open() call
>> explicitly specifies BDRV_O_NO_BACKING?
> BDRV_O_NO_BACKING is cleared in mirror_complete where the backing image is
> opened, we cannot use lock there as the backing is already open rw as source.

Oh, yes, of course you're right.

OK, then, if patch 7 of this series worked correctly, the bdrv_reopen()
in mirror_exit() would copy the locking status from the source BDS to
the target, effectively again overriding the disabled locking as it has
been set here.

I think the real issue is that mirror_complete() opens a completely new
backing chain, when it could just reuse the existing backing chain of
the source BDS. Unless you want to do that yourself, I guess I can take
a look into that, as this is very much related to my "block/mirror: Fix
target backing BDS" series.


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